PTMEG is a commodity in the chemical industry, widely used as a raw material in the preparation of polyurethanes.
One of the several methods of preparing PTMEG is the catalytic polymerization of tetrahydrofuran, using fluosulfonic acid (FSA) as the catalyst. PTMEG made this way sometimes has an undesirable yellow cast, which must be reduced or eliminated to meet most commercial specifications.
It has been found that this yellow cast is due, at least in part, to the presence of iron sulfites in the FSA. These sulfites are formed during shipment of the FSA in steel tanks by reaction of the steel's iron with the FSA's residual sulfur trioxide.
The yellow cast becomes apparent in the PTMEG when the iron sulfite concentration in the FSA used as catalyst is more than about 1200 ppm.sup.1. Preventing formation of the yellow cast is therefore a matter of bringing the iron sulfite content of the FSA to below that level before it is used. FNT 1. As measured by standard iodometric oxidation of a sample sparged with nitrogen, followed by a titration with thiosulfate to determine the amount of iodine consumed, and thereby the amount of sulfite present.
The iron sulfite content can be brought to below 1200 ppm by conventional methods such as distillation but it is far easier, and therefore preferred, to prevent iron sulfite formation in the first place by eliminating the FSA's residual sulfur trioxide through reaction with water according to the equation EQU SO.sub.3 +H.sub.2 O.fwdarw.H.sub.2 SO.sub.4